1. Field of the Invention
The present invention relates generally to an apparatus and method for allocating resources in a mobile communication system, and in particular, to a resource allocation apparatus and method for transmission of control information in a mobile communication system.
2. Description of the Related Art
Recently, mobile communication systems are evolving into advanced systems capable of providing not only the voice communication but also high-speed data communication while guaranteeing mobility for users. In light of such communication environments, Orthogonal Frequency Division Multiplexing (OFDM) and/or its similar Single Carrier—Frequency Division Multiple Access (SC-FDMA) are under study as a high-speed data communication scheme.
Presently, in 3rd Generation Partnership Project (3GPP) which is a standard group for asynchronous cellular mobile communication, Long Term Evolution (LTE), also known as Evolved Universal Terrestrial Radio Access (E-UTRA), evolved from the 3rd Generation (3G) mobile communication system is proposed as the next generation mobile communication system.
In addition, the LTE system is developing to a technology to which the OFDM and SC-FDMA technologies are applied. This multiple access scheme is characterized herein by allocating and managing time-frequency resources while maintaining orthogonality therebetween so that the time-frequency resources, over which it will carry data or control information for each user, may not overlap each other.
Meanwhile, in the LTE system, uplink control information includes Acknowledgement (ACK)/Negative ACK (NACK) information which is response information used for making a response to a success/failure in reception of transmitted downlink data, and Channel Quality Indication (CQI) information used for feeding back the downlink channel state.
The ACK/NACK information, which is generally composed of 1 bit, undergoes repeated transmission for improvement of reception performance and expansion of cell coverage. In a Multiple Input Multiple Output (MIMO) system to which multiple input/output antennas are applied, the ACK/NACK information is transmitted for each MIMO codeword. On the other hand, the CQI information is composed of a plurality of bits to express the channel state, and undergoes channel coding before its transmission, for improvement of reception performance and expansion of cell coverage. The block coding or convolutional coding scheme is available as a channel coding method for the CQI information.
A reception reliability required in receiving the control information is determined according to the type of the control information. Generally, a Bit Error Rate (BER) of a minimum of 10e−2˜10e−4 is required for ACK/NACK, while a Block Error Rate (BLER) of a minimum of 10e−2˜10e−1 is required for CQI.
In the LTE system, regarding the uplink control information, its transmission format is classified according to transmission/non-transmission of uplink data. When simultaneously transmitting data and control information over the uplink, the LTE system performs Time Division Multiplexing (TDM) on the data and control information, and maps the results to time-frequency resources allocated for data transmission before transmission thereof. On the other hand, when transmitting only the control information without data transmission, the LTE system uses an allocated particular frequency band for transmission of the control information.
According to the results made up to now in the standard conference, Physical Uplink Control Channel (PUCCH) is defined as a physical channel for transmitting the control information, and the PUCCH is mapped to the allocated particular frequency band. With reference to FIG. 1, a description will now be made of a detailed transmission structure for the PUCCH.
FIG. 1 is a diagram illustrating a transmission structure for a physical channel PUCCH for transmission of control information over the uplink in a 3GPP LTE system.
Referring to FIG. 1, the horizontal axis represents a time domain, and the vertical axis represents a frequency domain. The time domain corresponds to one subframe 102, and the frequency domain corresponds to a transmission bandwidth 110 of the system.
In the uplink, the basic transmission unit of the time domain is the subframe 102, and has a length of 1 ms. One subframe is composed of two 0.5-ms slots 104 and 106. The slot 104 (106) is composed of a plurality of SC-FDMA symbols 111˜124 (131˜144). For example, it is assumed in FIG. 1 that one slot is composed of 7 SC-FDMA symbols.
On the other hand, the minimum unit of the frequency domain is a subcarrier, and the basic unit of resource allocation is Resource Blocks (RBs) 108 and 109. The RBs 108 and 109 each is composed of a plurality of subcarriers and a plurality of SC-FDMA symbols. Herein, 12 subcarriers, together with 14 SC-FDMA symbols constituting 2 slots, constitute one RB.
The frequency bands, to which the PUCCH is mapped, are subcarriers corresponding to both ends of the transmission bandwidth 110 of the system, and they correspond to reference numeral 108 are reference numeral 109. The PUCCH can undergo frequency hopping to increase frequency diversity during one subframe, and in this case, slot-by-slot frequency hopping is available.
Shown in FIG. 1 is a structure for performing frequency hopping on a slot-by-slot basis as shown by reference numeral 150 and reference numeral 160. For example, control information #1, which was being transmitted over a pre-allocated frequency band 108 in the first slot 104, is transmitted over another pre-allocated frequency band 109 after undergoing frequency hopping in the second slot 106. On the contrary, control information #2, which was being transmitted over the pre-allocated frequency band 109 in the first slot 104, is transmitted over another pre-allocated frequency band 108 after undergoing frequency hopping in the second slot 106.
In the example of FIG. 1, in one subframe 102, SC-FDMA symbols for the control information #1 are transmitted as shown by reference numerals 111, 113, 114, 115, 117, 138, 140, 141, 142 and 144, while SC-FDMA symbols for the control information #2 are transmitted as shown by reference numerals 131, 133, 134, 135, 137, 118, 120, 121, 122 and 124. Further, SC-FDMA symbols for Reference Signals (RSs) are transmitted at times represented by reference numerals 112, 116, 139 and 143, or reference numerals 132, 136, 119 and 123.
Such RSs each are composed of a predetermined sequence, and used for channel estimation for coherent demodulation at a receiver. In FIG. 1, the number of SC-FDMA symbols for control information transmission, the number of SC-FDMA symbols for RS transmission, and their corresponding positions in the subframe are given by way of example, and they are subject to change according to the type of desired transmission control information and/or the system operation.
Meanwhile, regarding the uplink control information such as ACK/NACK information, CQI information and MIMO feedback information, Code Division Multiplexing (CDM) can be used to multiplex the uplink control information for different users, and CDM has a characteristic that it is robust against interference signals compared with Frequency Division Multiplexing (FDM). A Zadoff-Chu (ZC) sequence is under discussion as a sequence to be used for CDM-multiplexing the control information.
The Zadoff-Chu sequence, since it has a constant signal level (constant envelop) in the time and frequency domains, has a good Peak-to-Average Power Ratio (PAPR) characteristic and shows an excellent channel estimation performance in the frequency domain. In addition, the Zadoff-Chu sequence is characterized in that a circular autocorrelation for a Non-zero shift is 0. Therefore, in the case where the control information is transmitted using the same Zadoff-Chu sequence, User Equipments (UEs) can distinguish the transmitted control information by differentiating cyclic shift values of the Zadoff-Chu sequence.
In the actual wireless channel environment, the cyclic shift values are differently set for individual UEs intended to undergo multiplexing so as to satisfy the condition that they are greater than the maximum transmission delay value of the wireless transmission path, thereby maintaining orthogonality between the users. Therefore, the number of UEs capable of multiple access is determined from the length and cyclic shift values of the Zadoff-Chu sequence. The Zadoff-Chu sequence is applied even to the SC-FDMA symbols for RS transmission, making it possible to identify RSs between different UEs with the cyclic shift values.
Generally, a length of the Zadoff-Chu sequence used for the PUCCH is assumed to be 12 samples, the number of which is equal to the number of subcarriers constituting one RB. In this case, since the maximum possible number of different cyclic shift values of the Zadoff-Chu sequence is 12, it is possible to multiplex a maximum of 12 PUCCHs to one RB by allocating different cyclic shift values to the PUCCHs.
In this context, the LTE system applies cyclic shift values having an at least 2-sample interval between PUCCHs based on the frequency-selective channel characteristic. The application of the cyclic shift values having the at least 2-sample interval restricts the number of cyclic shift values in one RB to 6. In this way, it is possible to maintain the orthogonality between PUCCHs which are associated with the cyclic shift values on a one-to-one basis, without an abrupt loss thereof.
FIG. 2 illustrates an example of multiplexing CQIs for individual UEs with different cyclic shift values of the Zadoff-Chu sequence in the same RB in transmitting CQIs through PUCCHs with the structure of FIG. 1.
Referring to FIG. 2, the vertical axis represents cyclic shift values 0, 1, . . . , 11 (200) of a Zadoff-Chu sequence. The maximum number of channels that can undergo multiplexing in one RB without an abrupt loss of the orthogonality is 6, and shown in FIG. 2 is a case where 6 CQI informations 202, 204, 206, 208, 210 and 212 undergo multiplexing.
For transmission of the CQI information, the same RB and the same Zadoff-Chu sequence are used, and it is shown herein that CQI 202 from UE #1 is transmitted with a cyclic shift ‘0’ 214; CQI 204 from UE #2 is transmitted with a cyclic shift ‘2’ 218; CQI 206 from UE #3 is transmitted with a cyclic shift ‘4’ 222; CQI 208 from UE #4 is transmitted with a cyclic shift ‘6’ 226; CQI 210 from UE #5 is transmitted with a cyclic shift ‘8’ 230; and CQI 212 from UE #6 is transmitted with a cyclic shift ‘10’ 234. With reference to FIG. 1, a description will now be made regarding how to map the control information signals to the Zadoff-Chu sequence in CDM-transmitting the control information using the Zadoff-Chu sequence.
Let's say that for UE #i, a length-N Zadoff-Chu sequence is defined as g(n+Δi) mod N, where n=0, . . . , N−1, Δi denotes a cyclic shift value for UE #i, and i denotes a UE index used for identifying UE. Further, let's assume that the control information signal the UE #i desires to transmit is denoted by mi,k, where k1, . . . , Nsym. If Nsym indicates the number of SC-FDMA symbols for control information transmission in a subframe, the signals ci,k,n (indicating an nth sample of a kth SC-FDMA symbol for UE #i) mapped to the SC-FDMA symbols are defined as Equation (1).ci,k,n=g(n+Δi)modN·mi,k  (1)
where k=1, . . . , Nsym, n=0, 1, . . . , N−1, and Δi denotes a cyclic shift value of a Zadoff-Chu sequence for UE #i.
In the example of FIG. 1, the number Nsym of SC-FDMA symbols for control information transmission in one subframe is 10, excepting for 4 SC-FDMA symbols for RS transmission, and a length N of the Zadoff-Chu sequence is equal to the number, 12, of subcarriers constituting one RB.
From the viewpoint of UE, a cyclic-shifted Zadoff-Chu sequence is applied at every SC-FDMA symbol, and a control information signal it desires to transmit is formed in such a manner that one modulation symbol is multiplied by the time-domain cyclic-shifted Zadoff-Chu sequence at every SC-FDMA symbol for control information transmission. Therefore, a maximum of Nsym control information modulation symbols can be transmitted per subframe. That is, in the example of FIG. 1, it is possible to transmit a maximum of 10 control information modulation symbols during one subframe.
It is possible to increase multiplexing capacity of PUCCH transmitting the control information by further applying time-domain orthogonal covers in addition to the CDM control information transmission scheme based on the Zadoff-Chu sequence. A typical example of the orthogonal cover can include a Walsh sequence. For length-M orthogonal covers, there are M sequences satisfying orthogonality therebetween. Specifically, regarding 1-bit control information such as ACK/NACK, it is possible to increase multiplexing capacity by applying the time-domain orthogonal covers to the SC-FDMA symbols to which the ACK/NACK is mapped before transmission.
In the LTE system, PUCCH for ACK/NACK transmission considers using 3 SC-FDMA symbols for RS transmission per slot because of, for example, improvement of channel estimation performance. Therefore, in the example where one slot is composed of 7 SC-FDMA symbols, like in the case of FIG. 1, the number of SC-FDMA symbols available for ACK/NACK transmission is 4. By limiting a time interval where the time-domain orthogonal covers are applied, to one slot or shorter, a loss of orthogonality due to a change in wireless channels can be minimized. While length-4 orthogonal covers are applied for 4 SC-FDMA symbols for the ACK/NACK transmission, length-3 orthogonal covers are applied for 3 SC-FDMA symbols for the RS transmission. Basically, the ACK/NACK and RS can be subject to user identification with the cyclic shift values of the Zadoff-Chu sequence, and can undergo additional user identification by means of the orthogonal covers. For coherent reception of ACK/NACK, since there should exist RSs which are associated with ACK/NACK signals on a one-to-one basis, multiplexing capacity of the ACK/NACK signals is restricted by the number of RSs associated with the ACK/NACK signals.
For example, considering a maximum of 6 cyclic shift values per RB, since it is possible to apply different length-3 time-domain orthogonal covers for each cyclic shift of the Zadoff-Chu sequence applied to RSs, it is possible to multiplex RSs from a maximum of 18 different users. Since ACK/NACK signals are mapped to RSs on a one-to-one basis, it is possible to multiplex a maximum of 18 ACK/NACK signals per RB. In this case, there are four length-4 orthogonal covers being applied to ACK/NACK, and 3 of the orthogonal covers are used. The orthogonal covers being applied to ACK/NACK can be predetermined, or can be commonly recognized between UEs and a base station (also known as Node B) by signaling. Therefore, it is possible to increase the multiplexing capacity three times compared with the case where the time-domain orthogonal covers are not used.
FIG. 3 illustrates an example of multiplexing ACK/NACK for each UE with time-domain orthogonal covers in addition to different cyclic shift values of the Zadoff-Chu sequence in the same RB, in the above-stated PUCCH structure for ACK/NACK transmission.
In FIG. 3, the vertical axis represents cyclic shift values 300 of a Zadoff-Chu sequence, and the horizontal axis represents time-domain orthogonal covers 302. The maximum number of cyclic shift values with which multiplexing is possible in one RB without an abrupt loss of orthogonality is 6, and if three length-4 orthogonal covers 364, 366 and 368 are additionally used, it is possible to multiplex a maximum of 6*3=18 ACK/NACK signals.
Shown in FIG. 3 is an example where the same RB and the same Zadoff-Chu sequence are used for the ACK/NACK transmission in such a manner that ACK/NACK 304 from UE #1 is transmitted with a cyclic shift ‘0’ 340 and an orthogonal cover ‘0’ 364; ACK/NACK 306 from UE #2 is transmitted with a cyclic shift ‘0’ 340 and an orthogonal cover ‘1’ 366; ACK/NACK 308 from UE #3 is transmitted with a cyclic shift ‘0’ 340 and an orthogonal cover ‘2’ 368; . . . ; ACK/NACK 304 from UE #16 is transmitted with a cyclic shift ‘10’ 360 and an orthogonal cover ‘0’ 364; ACK/NACK 306 from UE #17 is transmitted with a cyclic shift ‘10’ 360 and an orthogonal cover ‘1’ 366; and ACK/NACK 308 from UE #18 is transmitted with a cyclic shift ‘10’ 360 and an orthogonal cover ‘2’ 368. The orthogonal covers 364, 366 and 368, which are length-4 orthogonal codes, satisfy orthogonality therebetween.
Meanwhile, for an arbitrary UE, in CDM-transmitting CQI or ACK/NACK over PUCCH, CQI and ACK/NACK that the UE desires to transmit may simultaneously occur in some cases. For example, when there is a need to transmit ACK/NACK in response to receipt of downlink data, transmission of CQI may occur.
At this point, the UE should simultaneously transmit the CQI and the ACK/NACK by applying pre-allocated cyclic shift value and orthogonal cover. In this case, multi-code transmission is performed, causing an increase in PAPR.